Most polygon mirror laser scanners commonly use an underfilled polygon, which means that the incoming beam to the polygon mirror is smaller than the individual mirror facets of the polygon. While the polygon is rotating, the beam position on the facet changes until the beam reaches the facet end. At that point, the laser is turned off and time is allowed for the beam to cross over to the next facet. Thus, the usable printing time (the duty cycle) is less than 100%, and is typically in the range of 60% to 70%. Most laser printers also are asymmetrical, meaning that the beam comes into the polygon at some large angle so that the beam shaping optics of the input beam do not interfere with the scanning optics.
Japanese Patent Application Laid Open Number 60-233616 discloses an underfilled symmetrical polygon printer. The printer uses a polarization method that is well known in other fields, such as optical disks, as a means for efficiently providing input beam and scanning beam separation. It discloses a symmetrical printer as a means for increasing the number of facets of the polygon, for the same polygon size and rotational speed, to thereby increase the effective writing speed in terms of lines per second. Employing a symmetrical configuration allows the use of a smaller polygon facet because the facet size is determined by the projected incoming beam.
Apparatus for this method is illustrated in prior art FIG. 1 where diverging light beams from laser 12 are collimated by lens 13 and further collected by cylindrical lens 14 to enter polarization beam splitter 10 as S polarized light. The incident optical beams are deflected 90.degree. in the beam splitter 10 and directed to the polygon mirror 8. The linearly polarized beams from the beam splitter 10 are formed into circularly polarized light in quarter wave plate 9, and then entered to the deflection mirror surface of the polygon 8. The beams reflected at the deflection mirror surface of the mirror 8 are formed into P polarized light through the quarter wave plate 9 transmitted through the polarization beam splitter 10 and then entered into the image focusing optical system 11 to scan the drum 7. The system is symmetrical in that the beam 15a scanned by mirror 8 is symmetrical about an optical axis 15.
U.S. Pat. No. 4,796,962 discloses a method for forming a symmetrical underfilled scanner by displacement of the incoming beam in the page direction. A symmetrical underfilled laser printer is disclosed in U.S. Pat. No. 4,284,994 which is achieved by using a pelical splitter. This has the disadvantage of a large loss of up to about 75% of the light as compared with a polarization beam splitter which is theoretically lossless.
FIG. 2 illustrates prior art polygon sizes as a function of the duty cycle for different configurations. In the asymmetrical configurations an angle of 90 degrees is assumed between the input beam and output beam at the center of scan. The full-width-half-power (FWHP) of the beam, at the media, on axis is 33.2 microns. The polygon size for an 8 facet asymmetrical underfilled polygon with duty cycle of 0.7 is 84.2 mm, while the diameter of the symmetrical underfilled polygon for the same duty cycle is 59.5 mm. Alternately, the polygon can be kept at about the same size while changing to the symmetrical configurations with more facets, such as a 10 facet polygon with a diameter of 92.9 mm.
Most polygon mirror laser scanners commonly use an underfilled polygon because overfilled polygons have several disadvantages. One disadvantage of the overfilled polygon is that it uses only a portion of the incoming beam power and thus has to use a more powerful laser as compared with the underfilled polygon. Another disadvantage is that the scanning beam diameter of an overfilled polygon printer is determined by the cross section of the incoming large beam and the facet. This means that it is proportional to the cosine of the angle between the incoming beam and scanning beam. Because the beam size at the media, in the scan direction, is inversely proportional to the diameter of the beam of the polygon, it too is dependent on the cosine term. This is a severe problem in asymmetrical overfilled polygon systems. Another disadvantage of overfilled printers is the power fall off caused by the nonuniformity of the large input beam.